There is a need in the food packaging industry for packaging film with improved structural integrity and which can be made into bags. Food products such as cheese, processed meat and raw meat are preferably packed by using film with high barrier resistance to moisture and oxygen. A typical way of protecting such food products from moisture and oxygen is by shrink packaging in high barrier film. A useful film with good moisture and oxygen barrier properties for this purpose comprises a copolymer of vinylidene chloride with a monomer such as methyl acrylate, vinyl chloride or other unsaturated monomer.
It is also known to provide a multilayer shrink film with a five layer structure, namely a first outer layer, a bonding layer, a barrier layer, a further bonding layer and a further outer layer, to achieve the desired properties. A known multilayer shrink film of this kind having good moisture and oxygen barrier properties for use in packaging food products has a barrier layer comprising a polymer such as a copolymer of vinylidene chloride, ethyl vinyl alcohol (EVOH) or other barrier polymer. It is necessary for the film to meet a number of criteria, such as being heat sealable, delamination resistant and providing a good moisture and oxygen barrier. The film should shrink from about 30 to about 50% at 90.degree. C. in both longitudinal and transverse directions and be capable of withstanding physical abuse during transportation. Other layers of the film are selected to provide requisite low temperature properties and abrasion resistance which are lacking in the barrier layer.
U.S. Pat. No. 3,741,253 (Brax et al) describes a multilayer barrier shrink film with a first outer layer of cross linked ethylene vinyl acetate polymer, a middle layer of a polymer of vinylidene chloride and a second outer layer of an ethylene/vinyl acetate polymer.
U.S. Pat. No. 4,892,765 (Hisazumi et al) describes a multilayer heat shrinkable film comprising a core layer of a copolymer of vinylidene chloride, outer layers of a polyamide and adhesive layers between the core layer and the outer layers.
U.S. Pat. No. 4,161,562 (Yoshikwa et al) describes a biaxially stretched five-layer film for packaging fatty food products of irregular shape. The film comprises a first outer layer of an ionomer, an olefin homopolymer or an olefin copolymer, or a mixture of the olefin homopolymer with the olefin copolymer, a second layer of an ethylene copolymer, a third layer of a vinylidene copolymer, a fourth layer of an ethylene copolymer and a fifth layer (second outer layer) of an ionomer.
In these prior patents, which are hereby incorporated herein by reference, a copolymer of vinylidene chloride comprises 65 to 95% by weight of vinylidene chloride and 5 to 35% by weight of at least one unsaturated monomer which is copolymerizable with vinylidene chloride, such as methyl acrylate or vinyl chloride. The copolymer is also blended with additives such as heat stabilizers and plasticizing compounds such as epoxyidized soybean oil and steramide known in the art. An amount of comonomer with vinylidene chloride is used to facilitate processability of the polyvinylidene chloride (PVDC) copolymer. However, higher amounts of the comonomer in the copolymer lower the barrier properties of the PVDC layers, and a high percentage of PVDC results in the copolymer becoming brittle and susceptible to cracking.
Yoshikawa et al teach that a PVDC layer in the multilayer film is likely to crack if the thickness of the PVDC exceeds about 15 microns due to low temperature brittleness of this polymer. In fact, cracking in a layer is a general problem in the manufacturing of multilayer film, especially when utilizing rigid or easily degradable polymers such as PVDC or EVOH. The thickness of such a barrier layer therefore has to be limited to minimize the likelihood of cracking.
A primary object of the invention is therefore to provide a multilayer plastic film with good barrier properties for moisture and oxygen and which is at least substantially free from the problems referred to above, as well as being substantially free from fine pin holes and stress cracking of barrier polymers, especially with greater thicknesses and also especially when VDC content is greater than about 95%. A relatively thick polymer layer is much more brittle than a copolymer layer with 80 to 90% VDC monomer.